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The objective of this study was to evaluate the effects of different times of exposure to heat on bone parameters and organ morphometry of Cobb Slow® and Hubbard Flex® broilers exposed to high temperatures for 1, 2 or 3 h daily, from 14 days of age. A total of 1120 one-day-old male chicks (half of each line; average weight of 44 g) were housed into 32 cages with 35 birds each, in a randomized-block experimental design. From the first to the 13th day of age, the birds remained under the thermal conditions recommended for their lines; thereafter, from the 14th day, the shed was separated into four parallel sections of 5.60 × 10.20 × 2.8 m delimited by double-sided plastic curtains along the width of the shed. Cyclic heat stress was generated by average temperatures of 36 oC from 14-20 days, 35 °C from 21-27 days, 34 °C from 28-34 days and 33 °C from 35-42 days. The total length, width, density and mineral matter of the tibia; the size of the duodenum, jejunum and ileum; and the weights of the bursa of Fabricius and spleen were measured. No interaction effect between thermal environment and line was observed. Heat stress for up to 3 h daily, from the 14th to the 41st day of age, led to a longer but narrower tibia. There was no influence on small intestine morphometry. The Hubbard line presented longer and wider tibiae as well as higher relative and absolute weights of the bursa. In conclusion, heat stress from 2 h daily modifies the structure of the tibia in broilers, which increases in length and reduces in width, but heat exposure has no influence on the small intestine or lymphoid organs. The Hubbard line shows better tibia development than the Cobb line as well as different primary lymphoid responses, represented by a heavier bursa.

Bone parameters and organ morphometry of two broiler chicken lines exposed to heat for different periods